As plasterers tinker with customized admixtures, pozzolans remain an intriguing ingredient.

Engineered to convert calcium hydroxide, otherwise known as lime, into harder substances, pozzolans hit the industry in the early 1990s.

“We use them for [durability] more than anything,” says Kirk Chapman, owner of Poolscape Unlimited in Lakeside, Calif. “It also makes the material a little creamier, and it’s a little easier to get a better finish.”

Because of its dark coloring, fly ash has largely fallen out of favor. But silica fume and metakaolins remain popular choices for pozzolans, particularly in the summer, when plaster requires a retarding agent to improve workability and durability.

The National Pool Industry Research Center at Cal Poly in San Luis Obispo, Calif., continues to experiment with admixtures of polymers and pozzolans in various combinations to determine the most durable blends.

NPIRC reported a 27 percent reduction in etching deterioration for pozzolan admixtures under aggressive water.